Known in the art is a method of preparing refractory articles from a chromium-containing material (SU, A, 678042) which comprises the mixing of a chromite ore, finely ground baked magnesite, and a binder, for instance, lignosulphonate, roasting of the prepared mixture at a temperature of from 1600.degree. to 1700.degree. C., fractionation of the baked material, preparation of a charge from 60 mass % of chrome-magnesite bake and 40 mass % of baked magnesite powder, molding of blanks from the charge, and roasting of the blank at a temperature of from 1700.degree. to 1800.degree. C. The mixing of the components is performed by multiple successive delivery of portions of the binder and baked dispersed magnesite into the grainy chromite ore, the amount of each of the components in the portion being 10-25% of the required quantity.
Also known in the art is a method of preparing refractory chromium-containing material in the form of an article (U.S. Pat. No. 4,169,734) which comprises the preparation of a mixture from fractionated powders of magnesite and chromite ore at a mass ratio of 30-90: 10-70, respectively, in the presence of 0.5-1.5 mass % of a binder selected from the group consisting of: MgCl.sub.2, MgSO.sub.4, and Ca(NO.sub.3).sub.2, or without a binder; molding of a blank from the prepared mixture in the presence of water under a high pressure; roasting of the blank at a temperature of from 1700.degree. to 1800.degree. C. to the article.
Likewise known in the art is a method of preparing refractory chromium-containing material (GB, A, 1510949) in the form of an article which comprises the preparation of a mixture from 26-56 mass % of a chromium-containing component, for instance, chromite, which is fractionated preliminarily to 0.5-5 mm (10-40 mass %) and 0.1 mm (60-90 mass %), with the fraction of the particles .ltoreq.0.06 mm in size being no less than 70% of the chromite mass in said mixture. Baked magnesite (MgO) is ground preliminarily to a particle size &lt;0.1 mm with the content of particle .ltoreq.0.06 mm in size equal to 70% of the magnesite mass; Blanks are molded from the mixture, and are subjected to a subsequent thermal treatment of the blanks at a temperature of from 1700.degree. to 1900.degree. C. for 4 hours to form the articles, for instance, bricks.
The known methods are multi-stage, power-intensive, and time-consuming. The prepared materials are characterized by refractoriness values not exceeding 1600.degree. C. which limits the field of application. Physical and mechanical characteristics of these materials are not constant and greatly depend on the dispersity of the initial components in the mixture which deteriorates essentially the quality of the articles manufactured from the above materials.
The principal object of the invention is to provide a method of preparing refractory chromium-containing material via a qualitative change of the composition of the mixture and thermal treatment conditions which will improve essentially physical and mechanical characteristics of the final material, simplify the technological process, and decrease energy consumption.
Said object is accomplished by a method of preparing refractory chromium-containing material comprising the preparation of a mixture from a chromium-containing component and a binder and thermal treatment of the mixture, wherein, according to the invention, as the binder introduced in an amount of 0.1-50 mass %, use is made of periclase, sand, clay refractory oxide, refractory scrap, wastes of blast-furnace processes, graphite production, and silicon carbide production, and inorganic salts taken separately or in a mixture. 8-20 mass % of a reducer is additionally introduced into the above mixture. As the reducer, use is made of at least one of the metals selected from the group consisting of aluminium, magnesium, silicon, zirconium, titanium and alloys thereof, after which the prepared exothermal mixture is subjected to thermal treatment under combustion conditions.
The use of the above-described method ensures the preparation of refractory chromium-containing material with refractoriness 1700.degree.-2000.degree. C., compression strength up to 70 MPa, porosity 14-56%, density 2.5-3.2 g/cm.sup.3, and characterized by uniformity of the properties over the whole bulk.
The mixture prepared by the above-proposed method represent an exothermal mixture able to burst into flame upon local initiation at any site with the aid of an electric coil, electric arc, or laser beam. The claimed ratio of the components of the exothermal mixture, as well as the combustion conditions, were chosen experimentally.
At the site of initiation (or inflammation) a wave of synthesis is generated which propagates with a bright glow along the whole bulk of the packed exothermal mixture, transforming said mixture into a strongly baked final refractory product. High temperatures arising from combustion ensure a complete reaction of the initial components and preparation of refractory material uniform over the whole bulk (with a small scatter of typical parameters). A deviation from the claimed ratio of the components results in the combustion front being attenuated and synthesis of the final product either does not occur at all or proceeds nonuniformly which then leads to deterioration of the quality of the final materials or articles.
The presence in the exothermal mixture of a reducer selected from the above metals or alloys thereof and taken in amount of 8-20 mass % ensures the inflammation of the mixture upon initiation which results in a simultaneous oxidation of the reducer and fast heat homogenization over the whole bulk of the initial mixture and, hence, in a high uniformity of the properties of the final material over the whole bulk. The content of the reducer is chosen according to the technological conditions, the nature of the initial components, and the qualitative composition of the final material. A lower content of the reducer decreases the efficiency of combustion and worsens the quality of the final product. A rise in the reducer content results in vigorous combustion, and ejection of the components, violates stoichiometry of the initial components, and deteriorates the quality of the final product because of melting and a change in uniformity of the composition. It is recommended to use as the reducer a mixture of at least two of the above metals taken at an equatomic ratio or an aluminum alloy selected from the group consisting of Al.sub.3 Mg.sub.2, Al.sub.3 Mg.sub.4, AlMg, Al.sub.2 Mg.sub.3, and TiAl. The combustion process becomes in this case more controllable, the quality of the final product can be more easily varied and its characteristics improved. Besides, the safety of the process enhanced, which is very important for industrial production. The use of Al-Zr and Si-Al alloys is also possible due to a similar effect. The metals can also be used as powders, wastes of aluminum and silicon industries, and sponge (titanium, zirconium).
As the chromium-containing component it is expedient to introduce into the exothermal mixture chromite (chromium ore), chromium oxide, chromium sulphate, chromate of one of the alkali metals selected from the group consisting of: calcium, barium, and magnesium, taken separately or in a mixture. The chromium-containing component is used predominantly in the form of powders 1-1000 .mu.m in dispersity and granules 1-5 mm in size; the fraction composition of the component can be varied both within and beyond the claimed limits depending on the type of the refractory material. For instance, for manufacturing large refractory articles dispersity of the chromium-containing component can be varied, according to the invention, from 1 .mu.m to 5 mm whereas for small articles (crucibles, tubes) and coatings from 1 to 500 .mu.m. The proposed method does not require preliminary fractionation of the powders, that is after the grinding of the initial components and determination of the total granulometric composition, the powders are directed for the production of the final products. Some deviations in dispersity of the chromium-containing components being used are also possible but in this case the content of the fraction 1 .mu.m must not exceed 20 mass %, and that of the fraction &gt;5 mm must be no more than 30 mass % of the granulometric composition of the chromium-containing components. Chromium compounds can be employed as slimes and wastes of the corresponding productions. Chromite is used when chromium oxide content is no less than 30 mass %.
The binder introduced into the exothermal mixture in amount of 0.1-50 mass % and having the above composition ensures a decrease in the combustion temperature, thereby providing the preparation of a high quality end product. The use of wastes of blast-furnace production, refractory scrap, wastes of graphite and silicon carbide production as the binder makes the process cheaper and influences favorably the ecological situation at the site of production due to utilization of wastes. Any deviation of the binder content in the mixture from 0.1-50 mass % decreases the efficiency of the combustion process which lowers the yield and worsens the quality of the final refractory material. It is recommended to use as the binder at least one refractory oxide selected from the group. Consisting of TiO.sub.2, ZrO.sub.2, MgO, CaO, Y.sub.2 O.sub.3, SiO.sub.2, and Al.sub.2 O.sub.3 or at least one inorganic salt selected from the group. Consisting of magnesium or aluminum sulphate, chloride, or nitrate, sodium silicate, and a mixture of at least two of said salts. The binder can be used in the form of powders with a dispersity of from 1 to 1000 .mu.m, fibers or whiskers from .ltoreq.1 .mu.m to 1 mm in diameter. The dispersity of the binders depends on the function of refractory materials, affects the characteristics of these materials, and is varied in a wide range from 1 .mu.m to 50 mm. For manufacturing large refractory articles, for instance, plates, according to the invention, coarsely dispersed binders with a particle size up to 50 mm can be used and for manufacturing crucibles or cups the dispersity of the binders must be from 1 to 1000 .mu.m. The fraction composition of a powderous binder from 1 to 1000 .mu.m is preferable and can be changed in any direction but the content of fractions &lt;1 .mu.m must not exceed 10 mass % and that of fractions &gt;1000 .mu.m must be no more than 20 mass % of the granulometric composition of the binder. In the process of preparing refractory material it can be shaped as required, that is the required article can be manufactured from the material. For this purpose, a blank is prepared from the exothermal mixture either by filling up a mold or by molding with the use of aqueous solutions of inorganic salts, that is water soluble fillers playing the role of binders. The use of wetted mixtures allows a coating to be made and an article of a complex configuration to be manufactured. Water is added in amounts required for preparing a liquid mobile mass. After molding the exothermal mixture in the form of an article or coating, water is removed by preliminary heating (drying). As the solvent use can be made of water, alcohol, water-alcohol, and water-acetone solutions. The heating of the packed exothermal mixture prior to thermal heating up to 100.degree.-800.degree. C. is needed for removal of the moisture and for a rise of the combustion temperature of weakly exothermal compositions and thus favors the preparation of a high-quality end material. Said temperature range depends on the composition of the exothermal mixture and can be changed towards high temperatures. The proposed method accomplished in the combustion mode is highly effective due to simple technology. The manufacturing of articles and coatings from refractory material can be performed in air without using reactors and furnaces, several syntheses can be accomplished simultaneously, and industrial application of the method does not require additional technological areas. The method is characterized by low energy-intensity, high capacity, and high quality of the prepared refractory chromium-containing materials and corresponding articles. The method makes it possible to obtain a high-quality coating on large surfaces with complex profiles. The use of wastes of blast-furnace and other processes as the binder favors a partial solution of waste utilization and ecology problems.